(1) Field of the Invention
The present invention relates to an unfired refractory composition. More particularly, the present invention relates to an unfired refractory composition which is moldable and hardenable at normal temperatures and which has an excellent mechanical strength over a broad temperature history range when it is used as a refractory in the unfired state.
(2) Description of the Prior Art
Fired refractory bricks have heretofore been prepared according to the method comprising pulverizing a refractory aggregate, adjusting the particle size if necessary, kneading the pulverized aggregate with an appropriate binder, molding the kneaded mixture into a desired shape, drying the molded mixture and firing the dried molded mixture to effect sufficient biscuiting. In order to obtain refractory bricks having a sufficiently high mechanical strength according to this method, it is necessary to carry out firing at such a high temperature as 1200.degree. to 1900.degree. C. for such a long time as 8 to 144 hours. Accordingly, this method is disadvantageous in that a large quantity of thermal energy is necessary for the firing operation.
Refractory bricks prepared by the process in which the firing step is omitted in the above-mentioned method so as to save thermal energy have already been known as unfired refractory bricks. More specifically, a binder capable of providing a sufficient strength at the molding step is incorporated in a refractory and the mixture is molded under high compression, and final firing of the so formed unfired refractory brick is effected by heat applied when the brick is actually used. As another unfired refractory, there is known an amorphous refractory. This amorphous refractory is a kneaded composition comprising an aggregate and a binder, which is used for repairing, protection or coating of a furnace, a laddle, a runner or other accessory member, and also in case of this refractory, final firing of the refractory is effected by heat applied when it is actually used in the furnace or the like.
However, when such unfired refractory is actually used for a liner of a furnace or accessory equipment, a problem described below arises.
The inner surface of this refractory liner which falls in contact with a molten metal or flame is heated at a temperature sufficient to sinter the aggregate, but the outer surface of the refractory lining is heated at a temperature of about 500.degree. C. at highest. In short, a large temperature gradient is formed in the thickness direction in the refractory liner. Ordinarily, a kneaded molded product comprising an aggregate and a binder has a certain mechanical strength at a relatively low temperature below 500.degree. C. by the action of the binder and at a high temperature exceeding 1200.degree. C., the product has a relatively high mechanical strength by sintering of the aggregate. However, in the intermediate temperature region between the above lower and higher temperatures, the strength is very poor. Accordingly, the above-mentioned refractory brick is readily broken in the region intermediate between the inner surface side and the outer surface side. This is a fatal defect of the known unfired refractory.
For example, sodium silicate which is most popularly used as the binder for the infinitive shape refractory has an appropriate sticking force and is advantageous in that it is excellent in the kneadability and moldability. However, an unfired refractory comprising sodium silicate as the binder is defective in that when it is heated at a temperature higher than 500.degree. C. but lower than the aggregate-sintering temperature, the mechanical strength is less than 1/3 of the mechanical strength of the unheated refractory and is less than 1/5 of the sintered refractory. Moreover, this unfired refractory comprising sodium silicate as the binder is difficult to harden at room temperature, and in order to harden this refractory, it is necessary to heat the refractory at a temperature higher than 250.degree. C. This heat treatment is troublesome. Fuerthermore, if this heat treatment is not conducted, the refractory comes to have a moisture-absorbing property and the strength of the unfired refractory is gradually degraded as it absorbs moisture. This is another fatal defect of the unfired refractory of this type.
Organic binders such as pitch, tar, atactic polypropylene, starch, phenolic resin and carboxymethyl cellulose have appropriate viscousness and adhesiveness. An unfired refractory comprising such organic binder has a satisfactory mechanical strength at low temperatures, but when it is heated at a temperature exceeding 500.degree. C., the carbonized binder is gradually oxidized and extreme reduction of the strength is caused as in case of the siliceous binder. Furthermore, when the unfired refractory comprising the organic binder is heated at the aggregate-sintering temperature, on carbonization of the organic binder, expansion or bubbling is caused to some extent. Therefore, it is very difficult to obtain sintered refractory having a fine and compact structure. This is another defect of the organic binder.
A phosphate type binder such as aluminum phosphate has a reactivity with most of refractory aggregates (which are essentially basic). If an aggregate is kneaded with such phosphate type binder, hardening takes place in a relatively short time, and even if the kneaded composition is not entirely hardened, a heterogeneous, partially hardened product or a premature hardening product is formed. Accordingly, an unfired refractory comprising a phosphate type binder is defective in that the working property is poor and the pot life is short. Furthermore, this unfired refractory is disadvantageous in that reduction of the mechanical strength in the above-mentioned intermediate temperature region is conspicuous.